Report consumption from run_event

`Event::consumes` says whether having run uses up what triggered it,
defaulting to no. `run_fn` already calls `should_run` per registration,
so it ors that across everything that ran and hands it back through
`run_event`. `CursorSenses` answers it with the sense it matched: a press
or a scroll is used up, hovering is not.

That drops `TypeEventManager::registered` and the second pass over a
widget's senses -- the match that decides consumption is now the same one
that decides whether the handler runs.

A cursor that is only resting still stops at the layer it is over, which
`run_event` cannot report because nothing need answer for it to be true.
It must not stop at a widget it has merely left, though, or ending a hover
above blocks the hover below: `leaving_a_widget_does_not_block_the_layer_below`
is that case, and it fails on `main` too.

core/src/event/manager.rs

@@ -135,19 +135,16 @@ impl<Rsc: HasEvents + 'static, E: Event> TypeEventManager<Rsc, E> {
         ));
     }
 
-    /// What this widget registered, without running any of it.
-    pub fn registered(&self, id: WidgetId) -> impl Iterator<Item = &E> {
-        self.map.get(&id).into_iter().flatten().map(|(e, _)| e)
-    }
-
     pub fn run_fn<'a>(
         &mut self,
         id: impl IdLike,
-    ) -> impl for<'b> FnOnce(EventCtx<'_, Rsc, E::Data<'b>>, &mut Rsc) + 'a {
+    ) -> impl for<'b> FnOnce(EventCtx<'_, Rsc, E::Data<'b>>, &mut Rsc) -> bool + 'a {
         let fs = self.map.get(&id.id()).cloned().unwrap_or_default();
         move |ctx, rsc| {
+            let mut consumed = false;
             for (e, f) in fs {
                 if let Some(data) = e.should_run(&ctx.data) {
+                    consumed |= e.consumes(&data);
                     f(
                         EventCtx {
                             state: ctx.state,
@@ -157,6 +154,7 @@ impl<Rsc: HasEvents + 'static, E: Event> TypeEventManager<Rsc, E> {
                     )
                 }
             }
+            consumed
         }
     }
 }

core/src/event/mod.rs

@@ -13,6 +13,14 @@ pub trait Event: Sized + 'static + Clone {
     fn should_run<'a>(&self, data: &Self::Data<'a>) -> Option<Self::Data<'a>> {
         Some(data.clone())
     }
+
+    /// Whether having run on this data uses up whatever triggered it, so
+    /// nothing further should see it. `run_event` reports back the `or` of
+    /// this across everything that ran.
+    #[allow(unused_variables)]
+    fn consumes(&self, data: &Self::Data<'_>) -> bool {
+        false
+    }
 }
 
 pub trait EventLike {

core/src/event/rsc.rs

@@ -21,12 +21,13 @@ pub trait HasEvents: Sized + UiRsc + HasState {
 }
 
 pub trait RunEvents: HasEvents {
+    /// Whether anything that ran consumed the input; see `Event::consumes`.
     fn run_event<E: EventLike>(
         &mut self,
         id: impl IdLike,
         data: <E::Event as Event>::Data<'_>,
         state: &mut Self::State,
-    ) {
+    ) -> bool {
         let f = self.events_mut().get_type::<E>().run_fn(id);
         f(EventCtx { state, data }, self)
     }